Sounds like they might
save you a lot of money short-term Chris!!
But seriously, I, too, have tried many and various solutions to this little problem over the years. The best to date is the pin hinge, but not the little 'Cabinet' type, the bigger, (3-4") version for hanging doors, coupled with a 'Cabin hook' as a back-stay to stop them flapping about. Another method that has worked to some degree, but that I have not yet fully perfected is to have a couple of blocks on the back face which fit around the uprights, These are reinforced with the small 'mending plates' (The little 90' brackets with holes available in most hardware shops) so that the last hole is 'proud' on the inside of the frame. This is to take the hook on a 'bungee' elastic, which then passes round the back two uprights of the frame, and back to the other wing board. The tension on this holds the wingboards in place. Mind you, the best idea that I've come up with yet is the one-piece proscenium! - The playboard, topboard support, (The topboard proper mounts into a couple of holes bored in this) and the side and wingboards all in one unit. The wingboards form doors on the front, which can
be used as weather /security protection 'on site' and are secured open by 'cabin hooks' which then double as stays. The beauty of this type is that the curtains can be a permenant fixture, and the whole unit, fitted with a handle on the top/bottom, (Personal preference) becomes a case for the dolls& props with either the backdrop or backcloth forming the other side. The hookboard can be hinged into the bottom edge as well, allowing this to be used to carry the dolls 'hung' if the handle is on the 'base'-(If this is the carriage method, The curtains will need securing at the bottom with elastic or similar, and should always be closed for carriage to protect the paintwork on the wingboards from damage) - I know this may sound complicated, but anyone capable of building it should be able to get the idea, I hope! - but in any event, I am always available to clarify points if required. Hope it is of help - Best of luck!